Why Large Dogs Die Young

When I was on vacation a couple of months ago, I posted a link to an article entitled "Why Small Pups Outlive Large Dog Breeds."

Fully Vetted reader 3Dogs1Cat (great handle!) asked for more information when the research was published in the April 2013 issue of American Naturalist. Here it is.

A 70 kg Great Dane has an average lifespan of about 7 years, whereas a 4 kg Toy Poodle can expect to enjoy a lifespan of about 14 years. This well-known pattern poses a conundrum for evolutionary biologists. Across species, large mammals live longer than their small counterparts. In marked contrast, within species, fast growth and/or large size seem to carry costs in terms of an individual's life span. This phenomenon has been documented not only in dogs, but also in mice, rats, and horses, and some have argued that lifespan even tends to be longer in humans with shorter stature.

Researchers have yet to determine why the patterns that we observe within species are opposite to those observed across species. In no species is the negative relationship between size and lifespan more evident than in the domestic dog. Artificial selection has led to breeds that range in body size from the 2 kg Chihuahua to the 80 kg Mastiff. Large breeds die at a median age of 5-8 years, whereas small breeds are expected to live on average about 10-14 years, i.e. twice as long. But why do large dogs die young?

To answer this question from a demographic perspective, Cornelia Kraus, Samuel Pavard and Daniel Promislow compared age-specific mortality in 74 breeds using data from more than 50,000 dogs, including their ages and causes of death, stored in the Veterinary Medical DataBase (VMDB). The authors hypothesize that larger breeds might have higher mortality rates due to the costs of increased, and suboptimal, growth rates. The question, though, is when those costs are paid. Do larger dogs live shorter lives because they have higher juvenile mortality, because their minimum or "baseline" mortality as young adults is increased, because they start aging earlier, or because the rate at which they age is faster?

The analyses show that the size-lifespan trade-off in dogs is mainly driven by a size-related speeding up of the mortality hazard. In fact, size affects many aspects of the mortality curve, but the strongest effect is on the rate of aging, which is positively correlated with breed size. Large dogs age at an accelerated pace, as though their adult lives are running at a faster pace than small dogs'. Hence, a first answer to the question of why large dogs die young is that they age quickly.

Future studies will need to determine the mechanisms behind these differences in mortality curves, and in particular, how the shape of mortality curves is determined by differences in underlying diseases that cause death. Dogs are a highly promising model to unravel the detailed evolutionary, genetic and physiological links between growth and mortality.